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reported in perm inches or grain/hft2in. Hg per inch of thickness. 1 perm inch = 1.46 x
Permeance. The rate of water vapor transmission per unit area at a steady state
through a membrane or assembly, expressed in ng/Pasm2 (grain/ft2hin. Hg).
Phenolic Plastics. Plastics based
on resins made
by the condensation
such as phenol and cresol, with aldehydes.
Pitch. See Incline; Coal tar pitch; or Petroleum pitch.
Plastic. A material that contains as an essential ingredient one or more organic
polymeric substances of large molecular weight. It is solid in its finished state and at
some stage in its manufacture or processing into finished articles can be shaped by
Plasticizer. Material, frequently solvent-like, incorporated in a plastic or a rubber to
increase its ease of workability, flexibility, or extensibility. Adding the plasticizer may
lower the melt viscosity, the temperature of the second order transition, or the elastic
modulus of the polymer.
Plasticizers. May be monomeric liquids (phthalate esters), low molecular weight liquid
polymers (polyesters) or rubbery high polymers (EVA). The most important use of
plasticizers is with PVC where the choice of plasticizer dictates under what conditions
the membrane may be used.
Ply. A layer of felt in a roofing membrane; a four-ply membrane should have at least
four plies of felt at any vertical cross section cut through the membrane.
Plywood. A flat panel built up of sheets of wood veneer called plies, united under
pressure by a bonding agent to create a panel with an adhesive bond between plies as
strong as or stronger than, the wood. Plywood is constructed of an odd number of
layers with grain of adjacent layers perpendicular. Layers may consist of a single ply or
two or more plies laminated with parallel grain direction. Outer layers and all odd
numbered layers generally have the grain direction oriented parallel to the long
dimension of the panel.
Polyester Fiber. Generic name for a manufactured fiber in which the fiber-forming
substance is any long chain synthetic polymer composed of an ester of a dihydric
alcohol and terephthalic acid. Scrims made of polyester fiber are used for fabric
Polyisobutylene. The polymerization product of isobutylene varying in consistency
from a viscous liquid to a rubberlike solid, with corresponding variation in molecular
weight from 1,000 to 400,000.
Polyisocyanurate. Thermoset polymer formed by polymerization of isocyanate; rigid
foam insulation meeting ASTM C1289-02 Standard Specification for Faced Rigid